The present invention relates generally to an angle-of-arrival (AOA) measurement via spectral estimation of radar time-of-arrival periodicities.
Many techniques for estimating the AOA of radio waves inpinging an array of antennas have appeared in the literature. See for example
A. Paulraj, R.Roy, T. Kailath, "Estimation of signal parameters via rotational invariance techniques - ESPRIT", Proc Nineteenth Asilomar Conf., Signals, Systems, Nov. 1985.
M. Wax, T.J. Shuan, T. Kailath, "Spatio-Temporal Spectral Analysis by Eigenstructure Methods", IEEE ASSP-32, No.4, August 1984, pp 817-827.
S. Kay, Modern Spectral Estimation, Prentice Hall Signal Processing Series, 1988.
A simple technique which uses a pair of antennas configured as shown in FIG. 1, is to measure the time-difference-of-arrival (TDOA) of a radar pulse. The TDOA for a signal at the two antennas is given by the following relationship. ##EQU1## where d is the distance between the antennas, c is the speed of light and .theta. is the angle of incidence (See J.B.Y. Tsui, Microwave Receivers with Electronic Warfare Applications, John Wiley & Sons, 1986). Thus if the TOA of a pulsed signal can be measured very accurately, this information can be used to calculate the AOA. An EW receiver ustilizes a TOA circuit to time tag radar pulses. This unit typically consists of a threshold detector (which changes its output state upon detection of a video pulse from the receiver) and a high speed digital counter. The counter's count value is read into memory when the threshold detector's output state changes.
There are two problem with this approach: 1) If the EW receivers are exposed to a single stable pulse train then the TOA pulse trains for antennas 1 and 2 appear as shown in FIG. 2. The TDOA is estimated by taking the time difference between the undelayed or reference channel and the delayed channel. In practice many individual pulse trains are interleaved in time and each individual pulse train may be staggered or jittered in time. Under these conditions the simple time differencing scheme fails. 2) In many cases accurate single measurements of the TOA are difficult to make due to the finite rise time of the radar pulses. The TOA values from one measurement to another appear jittered. This is one of the reasons this approach has not been used for short baseline systems (see the Tsui text cited above). The invention presented herein provides a technique to use the TDOA measurement in a multiple signal environment and uses a set of TOA measurements over which an averaging process occurs.
The following U.S. patents are of interest.
U.S. Pat. Nos. 5,053,784 - Hippelainen
4,888,593 - Friedman et al
4,768,036 - Litchford et al
4,500,884 - Naubereit et al
4,438,439 - Shreve
The closest patent is Naubereit which discloses an automatic aircraft carrier landing system in which an aircraft is guided to the carrier pitch and bank information from the carrier so as to indicate to the pilot of the aircraft whether he is within a prescribed flight path.
The Friedman et al patent relates to a direction finding method and apparatus for a radio signal source, modulated by a digital information signal and existing in a heavy interference environment. The direction finding technique exploits the second order periodicity of a transmitted signal having digital modulation. The signal time of arrival difference between two antennas is determined using a cyclic crosscorrelation method.
The Hippelainen, Litchford et al and Shreve patents are of less interest. None of the cited patents use a spectral estimation technique that extracts periodic Properties of radar signals whose time-of-arrivals have been tagged by an Electronic Warfare receiver.